I wish they had seals at the top nut/stem seam on threaded headsets. Jillions of cases of seized stems (including mine) could have been avoided if only someone thought of adding an o-ring seal at the top nut. Maybe I'll put in a little bit of clear silicone in that area to seal the seam (in addition to a little bit of grease on the stem itself) when I reinstall the stem on my Vitus Carbone 7. It will be invisible to the eye anyway.
Chombi
84 Peugeot PSV
85(?) Vitus Carbone 7 Plus